In some types of automatic workpiece processing, the workpieces are located and clamped in movable fixtures known as pallets, which are progressively transferred from station to station of a multiple station transfer machine. In each station of such a multiple station machine, these pallets are located as accurately as possible and clamped by a mechanism referred to as a pallet registry. Once located, a machine adjacent the pallet performs a milling or drilling or other operation on the workpiece. The accuracy of the resultant operation is only as accurate as the location of the pallet and workpiece.
A wide variety of types of registries have been in use in the industry for many years. Several such registry mechanisms are shown in U.S. Pat. Nos. 2,672,675; 2,673,386; 3,155,217; 3,571,872; 3,968,869 and my copending application, Ser. No. 950,318, filed Oct. 11, 1978. The location of a pallet with respect to a registry is generally accomplished with a pair of locating pins, vertically movable in the registry, which engage corresponding holes in each pallet. Slight errors of pallet location are inevitable because of the practical working clearances required between the locating pins and their guide sleeves in the registry frame, and between the locating pins and the corresponding holes in the pallet. These slight errors become progressively larger with usage due to pin, guide sleeve, and pallet hole wear.
It is one object of this invention to provide a pallet registry which is free of these initial working clearance type errors associated with locating pins, and, further to provide a pallet registry in which the location errors due to wear are significantly reduced.
It is often times convenient to rotate to pallet 90.degree. about a vertical axis at some intermediate station as a pallet moves through the machine. This is generally done to bring otherwise inaccessible workpiece surfaces into position for processing by working stations which are disposed along either side of the line of pallet travel.
It is another object of this invention to provide a pallet registry and pallet combination which is capable of precisely locating a pallet with respect to the registry in any one of four attitudes of the pallet which are created by rotation of the pallet in 90.degree. increments about a vertical centerline.
The movement of pallets through a transfer machine is presently accomplished in one of two ways; they can be slid from station to station on fixed rails driven by a horizontally reciprocating transfer bar which selectively engages all pallets to move them in the forward direction, and is disengaged from the pallets during its return travel; or the pallets ca be moved by a "lift and carry" mechanism from station to station by first being lifted by a set of transfer bars in a substantially vertical direction, then being moved forward with the transfer bars through a horizontal stroke equal to the station spacing, and finally being lowered by the transfer bars in a substantially vertical direction. The first slide system of pallet transferring requires only a simple reciprocating transfer bar and slide rails but has the disadvantage of locating the pallets along the vertical axis from the rails on which the sliding occurs and through the pallet feet which slide on the rails. Both the rails and pallet feet are very subject to wear, especially where dirt, dust and grit can accumulate on the rails, and this combined wear directly affects the accuracy of vertical pallet location. The lift and carry pallet transfer system eliminates the vertical location error problem due to wear, but at the expense of incorporating a more complex lift and carry type transfer system for the pallets.
It is another object of this invention to provide a pallet registry which combines the simplicity of moving the pallets on slide rails using a reciprocating transfer bar, but vertically locating the pallet in the registry on surfaces not subject to sliding wear.
Other objects of this invention are to provide registries which may be grouped together to be operated by a single power source; to provide registries in which the clamps have a significantly greater movement range than those of present designs, to provide registries which do not utilize screws, wedges, or other low efficiency mechanisms in generating the clamping force, and to provide registries in which the bending loads due to clamping are primarily confined to members whose deflection does not affect the accuracy of pallet location, and in which the bending loads imposed on the locating and structural components of the registry are minimized to achieve lower deflections per unit of clamping force.
Other objects of this invention will be apparent in the following description and claims with the accompanying drawings in which there is disclosed the principles of operation of the invention and the best mode presently contemplated for the practice thereof.